I. Field of the Invention
This invention relates to a method and apparatus to encapsulate, using a liquid resin, a surface and joints between conducting leads and contact pads on the surface of a semiconductor device.
II. Description of the Prior Art
Encapsulation of semiconductor devices including portions of the devices having terminal leads is desirable in terms of improving the reliability of the finished package. It is generally desirable to employ a liquid resin on the surface of the semiconductor device and the joints which exist between the conducting leads and the contact pads on the device surface. A variety of techniques exist in the art to provide conductive leads to semiconductor devices. Typical are devices with leads bonded to contact pads, for example, contact pads existing on the semiconductor chip, or contact pads existing on a chip substrate.
Typically, a large number of metallic interconnections must be coupled to an individual chip about its perimeter. One technique is known as tape automatic bonding (TAB) which employs a roll-type film cut to form an individual set of chip leads. This individual set is formed by utilizing etch techniques to obtain a personalized lead pattern which corresponds to a give chip terminal pattern. The etch pattern is employed to create a large number of identical sets. The sets are introduced one by one at the bonding station for affixation. Reference is made to U.S. Pat. No. 4,551,912 which discloses an integrated tape bonding system. Using these techniques, all leads to the chip are connected simultaneously. By employing a continuous reel transport process system through-put is enhanced. TAB is therefore very useful for achieving high production rates to couple chips to identical I/O circuitry.
Another technique involves providing leads which are bonded to a lead frame. In these devices, the semiconductor is first attached to a pad of a lead frame. This is generally recessed and located in the center of the lead frame pattern. Contact pads on the semiconductor device are then individually attached by wire bonding or by tape automated bonding to corresponding contacts on the ends of the leads of the lead frame adjacent to, but spaced from the mounting pad.
Encapsulation of the device coupled to the lead frame is known in the art as a technique providing a finished package. Typical are then encapsulation techniques disclosed in U.S. Pat. No. 4,504,435 and 4,641,418. More generally, packaging techniques for encapsulating electronic modules utilizing plastic materials which extend to the device/contact lead are disclosed in U.S. Pat. Nos. 3,668,299; 3,930,114; and 4,017,495.
One of the problems with prior art systems is the inability to prevent liquid from flowing onto the terminal portions of the leads which themselves extend in a cantilevered fashion beyond the surface edges of the device. This spreading occurs subsequently to the application of the drop of liquid polymeric material which was applied to the surface of the chip which has been bonded to a frame or an automated bonding tape. This liquid polymer is applied solely to the chip surface and ideally, should spread out over the die surface and cover only the inner bond leads. However, the materials of choice being of a low viscosity tend to form a thin coating which wicks-out along the leads. Ultimately, such a coating extends beyond the inner lead bond areas and may travel as far as the outer lead bond areas. Such is highly undesirable since it interferes with subsequent tape excising and substrate bonding operations. More importantly, this causes variability in the resulting thickness of the finished encapsulation, and may introduce undesirable stresses due to the reduced flexibility of regions of the TAB package.